1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, facsimile apparatus, printer, or an apparatus that combine these, and more specifically relates to a developing device utilized in these apparatuses and to a process cartridge that has this developing device.
2. Description of the Related Art
In the past, developing devices were used in order to develop an image carrier surface by following an electrostatic latent image pattern formed on the image carrier surface. Concretely, the developing device had a toner housing chamber for housing toner, a developing roller provided in the toner housing chamber for supplying toner to the image carrier, a toner supply roller provided in the toner housing chamber for supplying toner to the developing roller, and toner layer regulating means for forming a thin layer of toner on the developing roller by making contact with the developing roller; and had the function of developing an electrostatic latent image on the image carrier by applying bias voltage to the developing roller.
In order to obtain a high quality image without concentration irregularities, the amount of toner adhering to the developing roller had to be sufficient to form the toner image, and that the toner had to be uniformly charged. However, in one-component non-magnetic developing devices using non-magnetic toner, toner could not adhere to the developing roller by magnetic force because no magnetic body was contained, and therefore, the amount of adhering toner was prone to be insufficient.
In order to resolve this issue of an insufficient amount of adhering toner, proposed in Japanese Unexamined Patent Application Publication H 5-273848, for example, is a developing device composed of a toner supply roller with a foam surface such that toner can be copiously supplied to the developing roller, and toner is coated on and supplied to the developing roller surface by the developing roller making contact with the toner supply roller that contains a large amount of toner. More concretely, by making the foam density in the range of 0.18 to 0.28 g/cm3 and making the amount of compression deformation of the toner supply roller with the contact part of the developing roller be in the range of 0.2 to 1.5 mm, this is a technology that can guarantee satisfactory supply performance while avoiding increased torque caused by excess pressure contact.
Moreover, disclosed in Japanese Unexamined Patent Application Publication H 8-220872 is a technology that, when a toner supply roller composed of foam rotates while contacting the developing roller, toner contained inside the foam cells is blown out without being cut off evenly by a toner layer formation member, and generation of images with stripe-shaped light and dark irregularities on the image is prevented. More concretely, a developing device was proposed that prevents images with light and dark irregularities by mathematically formulizing the relationship between the toner fluidity index and such toner supply roller conditions as cell density and amount of indentation.
However, in the conventional developing devices disclosed in the patent publications described above, uneven contact is caused by the contours of the foam cells of the toner supply roller, and the toner supply roller generates scraping irregularities on the thin layer of toner already adhering to the developing roller, and induces triboelectric charge irregularities. This phenomenon tends to occur when the amount of toner saturation charge increases and the electrostatic adsorption force of the toner onto the surface of the developing roller is heightened, such as in low humidity environments. The toner newly supplied when toner supply roller has made scrapes has insufficient charge and also has weak electrostatic adsorption force, and therefore the developing characteristics of the image carrier are uneven, and light and dark noise corresponding to the contoured pattern of the cells appears on the image.
Technologies relating to the present invention are also disclosed in e.g., Japan Patent No. 3197934.